I'd say it's best to replace the compressor. No point in trying to free it up. Don't know what kind of friction it will be pulling if you do free it up.

 

Yeah, good point... thanks guys!

 

If it's stock it's R12. Have to do a total refit.

 

Right if it hasn't been converted there may be r12 in there and they aren't compatible with each other. The engine dying at idle means the compressor is sticking. It needs to have oil. If it hasn't run in a long time the oil is likely bad. If you run the compressor without oil it will seize up and break apart. I would replace the compressor if its failing. Check for contamination in the lines. You should also replace the receiver ( the canister looking thing) and the orifice (in the line after the cooler). The received holds the coolant and oil and will be contaminated if anything broke up on the pump. The orifice is a little reduction tube with a screen that regulates the pressure. It gets clogged. Stop leak clogs up everything especially in a dirty system. The parts arent real expensive and you will be glad you replaced them when your ac is nice and cold. When you start up everything you want to draw a vacuum to remove any air and boil off any moisture. You need to add the proper pag oil because there won't be any in the system. Then you need to properly fill the system. This is a great time to switch to R134a. If you don't have the proper tools I would take it to a shop and tell them about the new parts so they can properly add the oil and charge the system. It should be under $100 and will guarantee the life of your new parts. Alternatively you can buy the tools for around the same price from Harbor Freight and charge more systems. Of course it is illegal to evacuate the system into the atmosphere.